Miniature microwave isolator for strip lines



Oct. 13, 1970 N. EBERHARDT 3,534,299

ummunn MICROWAVE xsomoa FOR STRIP muss 2 Sheets-Sheet l VARIES ABOUTRESONANCE Filed Nov. 22, 1968 44 FIG. 4A

l2--'Tm!!-"' /42 IN l ENTOR By N. EBERHARDT A 7' TORNEV Oct. 13, 1970 N.EBERHARDT 3,534,299

MINIATURE MICROWAVE ISOLATOR FOR STRIP LINES Filed NOV. 22, 1968 2Sheets-Shem 2'! INTERNAL BIASING FIELD PERMEABI LITY FORWARD. DIRECTIONR.F. MAGNETIC FIELDS WIDE DIMENSION 0F CONDUCTIVE CHANNEL 3,534,299MINIATURE MICROW VE ISOLATOR FOR STRIP LINES Nikolai Eberhardt,Bethlehem, Pa., assignor to Bell Telephone Laboratories, Incorporated,Murray Hill, NJ,

a corporation of New York Filed Nov. 22, 1968, Ser. No. 778,148 Int. Cl.H0111 1/32 US. CI. 33.3-24.2 6 Claims ABSTRACT OF THE DISCLOSUREBACKGROUND OF THE INVENTION This invention relates to microwave stripline devices and more particularly to nonreciprocal gyromagneticattenuating devices of the general class known as resonance isolators.

The operating principles of resonance isolators in conductively boundedWaveguides of the usual dimension are Well known and many versions havebeen described for which different advantages have been asserted. Ingeneral all of these devices involve an element of gyromagneticmaterial, such as ferrite, located in the field pattern ofelectromagnetic wave energy and magnetically biased to the point atwhich the material becomes resonant in a gyromagnetic sense at thefrequency of the energy. In a certain plane parallel to one of thenarrow walls, the wave energy has a transverse magnetic field componentand a longitudinal magnetic field component that are equal in amplitudeand 90 out of phase so that the magnetic field is circularly polarizedand appears to rotate in one direction for propagation along the guideand in the opposite sense for propagation in the opposite direction.When this rotation is in the sense defined as positive with respect tothe direction of the biasing field, there is strong coupling to thematerial and high dissipation of energy. When negative, there is littlecoupling and little dissipation. However, the region of purecircularization is usually narrow in width so that within the resonantmaterial there are minor field components that rotate in a senseopposite to the desired sense and dilute the nonreciprocal effect,usually by increasing the forward loss.

Several proposals have been made for inhomogenously loading the guidecross section and/or inhomogenously biasing the material according toparticular profiles to reduce the forward loss. These proposalsgenerally attempt to increase the ratio of forward propagating purelycircularly polarized components rotating in the noncoupling sense tothat of the components rotating in the opposite and coupling sense. Thestructures resulting from these proposals have been in general toocomplicated, large and bulky to be integrated directly with striptransmission lines. Typical of this prior art are the disclosures of W.W. Anderson et al. in Pat. 3,051,908 granted Aug. 28, 1962 or F. S. Chenin Pat. 3,142,026, granted July 21, 1964. Isolators in reduced sizewave-guides using heavy gyromagnetic loading have also been proposed,but these structures have had high forward losses. Typical of the latterprior art is the disclosure of H. Seidel et al. in 38 BST] 1427,November 1959.

3,534,299 Patented Oct. 13, 1970 SUMMARY OF THE INVENTION In accordancewith the present invention it has been recognized that if a shortsection of very small crosssectioned conductively bounded rectangularwaveguide channel is completely filled with ferrite and biased by amagnetic field that increases across the wide dimension from a strengthnear one narrow wall that is substantially below that for resonance atthe frequency of interest, through the strength for resonance at aparticular intermediate point, and to a value above that for resonancenear the other narrow wall, an isolator is produced which simultaneouslyhas an unusually low forward loss, and a field distribution and sizewhich are compatible with strip lines. In a particular embodiment theisolator comprises simply a small block of ferrite, no more than afraction of the corresponding air filled waveguide dimension for thesame frequency, plated with conductive material or wrapped in conductivefoil that covers four of its sides to form a miniature heavily loadedwaveguide. This guide or conductive channel is disposed between thestrip line center conductor and one or the other, or both, of the stripline ground planes. Pole pieces of simple design provide a biasing fieldof the profile desired. It has been found that this biasing field sodistorts the magnetic field pattern of the high frequency wave withinthe ferrite loaded foil boundary that the longitudinal and transversecomponents are equal in amplitude in a relatively broad region thatsubstantially coincides with the region of resonance. The biasing fieldis directed in the noncoupling sense for forward transmission throughthis region to obtain low forward loss. For the reverse direction, highreverse loss is obtained since the gyromagnetic filling factor is high.It has been further found that this distorted field is one to whichstrip lines can be well matched.

BRIEF DESCRIPTION OF DRAWING FIG. 1 is a cutaway perspective view of astrip line embodiment in accordance with the invention;

FIGS. 2A, 2B, and 2C, given for the purpose of explanation, showsignificant parameter variations across a cross section of FIG. 1;

FIG. 3 illustrates by means of a cross-sectional vieW one means ofapplying the required biasing field to and certain other modificationsof the embodiment of FIG. 1;

FIG. 4 is a longitudinal cross section and FIG. 4A a transverse crosssection of a further embodiment of the invention; and

FIG. 5 is a longitudinal cross section and FIG. 5A a transverse crosssection of an unbalanced version of the invention.

Referring more particularly to FIG. 1 an isolator in accordance with theinvention is shOWn in combination with a section of microwave strip lineof the type which supports a wave usually designated as a TEM mode. Thestrip line itself is conventional and comprises a pair of flat, spaced,conductive ground planes 10 and 11 together with an interposed centerconductor 12. While these elements are shown in a selfsupporting form itis understood that they may be fabricated in the familiar sandwichfashion in which a dielectric substrate is included between one or bothof the ground plates and the center conductor, and in which the centerconductor itself may be formed by printing, plating or etching a thinnarrow conductive layer on one or the other of the substrates. Since thetransverse extent of ground planes 10 and 11 are immaterial, they aretruncated to illustrate that the elements shown may comprise a smallpart of a larger integrated strip line package.

The isolator comprises a pair of conductivel bounded channels 16 and 17located respectively above and below center conductor 12 each completelyfilled with gyromagnetic material. In the form illustrated the outsidewide conductive boundaries of each channel comprise the ground planesand 11 and the side or narrow boundaries comprise conductive partitions13 and 14 extending between and fastened to the ground planes. Aconductive center divider 15 lies in the plane of and is connected tocenter conductor 12 and separates the two channels.

The material within each channel thus formed is of the type havingelectrical and magnetic properties of the type described by themathematical analysis of D. Polder in Philosophical Magazine, January1949, Volume 40, pages 99 through 115. More specifically, it may be madeof any noncondueting ferromagnetic material. For example, it maycomprise iron oxide with some of the oxides of one or more bivalentmetals such as nickel, magnesium, zinc, manganese, or aluminum, combinedin a spinel crystal structure. This material is known as ferromagneticspinel or as ferrite. Since a narrow resonance line width is preferredfor the invention as will be discussed hereinafter, it may comprise oneof the ferromagnetic garnet materials. Any of these materials aresometimes first powdered and then molded with a small percentage of aplastic binder. Hereinafter the term ferrite will be used exclusively asdescriptive of the material, but it will be understood that equivalentmaterials having similar gyromagnetic properties may be used to practicethe invention. It will also be convenient to refer to the ferritefilling as well as to the conductively bounded channels merely by thereference numerals 16 and 17.

The relative sizes of channels 16 and 17 may be given in terms of thefree space wavelength of the frequency of interest. Since the channel isso heavily loaded by the ferrite, the wide dimension of each channel maybe only in the order of .15 to .2 free space wavelengths (as compared to0.5 to 0.75 wavelengths for a standard waveguide) and a narrow dimensionin the order of .02 to .05 free space wavelengths (as compared to 0.25wavelengths in the standard waveguide). Thus at a frequency of 6 gHz.the ground planes are typically spaced 0.122 inch, each channel has anarrow dimension of 0.0625 inch and a wide dimension of 0.38 inch. Thelength of each channel is a few tenths of an inch depending upon theisolation to be introduced.

The material within each channel 16 and 17 is biased by a steadymagnetic field schematically represented by the vectors H directednormal to ground planes 10 and 11 and having a strength which increasesacross the channels as will be shown in connection with FIG. 2A. ThusFIG. 2A illustrates by means of the characteristic the variation ofinternal biasing magnetic field across the wide dimension of eitherchannel 16 or 17. Near one narrow wall the field has a very small value.At a point displaced from the center line of the channel and near theother narrow wall the field increases through the value w/'y, thewell-known strength producing gyromagnetic resonance at the operatingfrequency m where v is the gyromagnetic ratio. Beyond this point thefield continues to increase. Good performance has been obtained when thelocation of the resonance region falls between 15 and 25 percent of thechannel width away from the nearer narrow wall. The curvature ofcharacter istic 25 is the result of demagnetizing factors inherent withthe simple pole pieces to be described and does not appear to besignificant. FIG. 2B illustrates the significant permeability componentsof the ferrite produced by the magnetization of FIG. 2A. Thesecomponents are well known and a full definition and description of themmay be found in a text book such as Microwave Ferrites andFerrimagnetics by Lax and Button (1962) or in articles such as Behaviorand Application of Ferrites in the Microwave Region by A. G. Fox et al.,34 EST] 5, January 1955. Thus the value ;t'(), the real portion of thepermeability for negatively circularly polarized components, remainsslightly greater than unity regardless of 4 the field strength. Thevalue ;t'(l), the real part of the permeability for positivelycircularly polarized components, is approximately unity over most of thecross section, sharply becomes negative as resonance is approached,passes through zero at resonance to positive values above resonance, anddecreases for higher field values. The imaginar or loss component of thepermeability for positively circularly polarized components is shown bythe curve n(+) which is zero over most of the cross section but risessharply to a maximum at resonance. The width of the loss characteristicis known as the resonance line width and is a property of the particularmaterial. FIG. 2C illustrates the relative values of transverse andlongitudinally radio frequency magnetic field components H and Hrespectively. For comparison the broken line curves 26 and 27 illustratethe usual transverse and longitudinal components in a waveguide ofsymmetrical cross section and are also typical of the distribution ineither channel in the absence of the biasing field H The solid curves 28and 29 represent the respective distribution of the transverse andlongitudinal components as determined experimentally by field probingtechniques when the biasing field according to FIG. 2A is applied. Amathematical analysis of the boundary conditions responsible for thisdistribution would add little to an understanding of the invention andis not offered. What is significant, however, is that the biasing fieldprofile of FIG. 2A produces in the region where the field strengthpasses through the resonance value, the loss characteristic p."(+) ofFIG. 2B as well as a region in which H and H are substantially equal asillustrated by the region 30 on FIG. 2C. Thus a region of circularpolarization exists which appears to have a width very nearly equal toand coinciding with the resonance linewidth for positive circularpolarization. When the sense of the biasing field H is negative withrespect to this circular polarization produced by one direction ofpropagation, there is for this direction little absorption. In a typicalembodiment the loss thus introduced has been no greater than 0.7 db.Reversing the direction of propagation reverses the sense of circularpolarization. It cannot be determined by field probing techniquesexactly what is the reverse field distribution since the components areabsorbed and dissipated in such a short length of material, but thedistribution is clearly one in which substantial positively circularlypolarized components exist in region 30. In a typical embodiment it hasbeen found that this loss is in the order of 10 db for each 0.1 inch ofchannel length, the large magnitude being due to the high filling factorof the ferrite. Thus a channel length of only a few tenths of an inchproduces a dissipation equivalent to that of much longer prior artisolators.

FIG. 3 illustrates by means of a cross-sectional drawing a preferedmodification of the structure of FIG. 1 and corresponding referencenumerals have been used to designate corresponding components.Modification will be seen to reside in the simplified form of theconductive boundary for ferrite elements 16 and 17 obtained by way ofthe expedient of wrapping layers of conductive foil 31 and 32 about eachof ferrite blocks 16 and 17, respectively. The wide dimension of eachfoil contacts a ground plane 10 or 11 on one side and the centerconductor 12 on the other. Thus the foil wrapped blocks of ferrite canbe directly introduced into the existing space between the centerconductor and the ground planes to form an isolator in an integratedstrip line assembly.

FIG. 3 also illustrates the simple cross section of pole pieces 33 and34 to produce the field distribution of FIG. 2A. The angle ofapproximately 57 degrees incline of the pole piece face has beendetermined experimentally to give good results for the particularferrite and geom etry used and will in general be dependent upon thematerial properties and geometry of the ferrite.

Obviously the match between the strip line and the conductively boundedchannel must be good to eliminate a loss introducing impedancediscontinuity at this point. In general the transition may be the sameas any other strip line to waveguide transition and the numeroustransducers already known to the art may be used in miniature form tomake the junction.

For the particular transition illustrated in FIG. 1 it is preferred thateach channel be located on center conductor 12 so that the strip linecouples with each channel in a region displaced from the channel centerline on the low biasing field side thereof. This location tends tocouple maximum transverse fields on the strip to maximum transversefields in the channel and to match the respective electric fields. Theprecise point of match depends upon the contour of the biasing field andis best determined experimentally. The match is further improved bycapacitive stubs 18 and 19 on center conductor 12. Other impedancematching aids such as capacitive posts, screws or tapers may be used toimprove the match as required.

A further transition is shown in FIG. 4 and the transverse cross sectionthereof in FIG. 4A in the form of a dielectric waveguide 41 comprising anarrow strip of material having a high dielectric constant that overlapscenter conductor 12 at one end and extends into the channel region atthe other. The end of dielectric Waveguide 41 that overlaps centerconductor 12 can be tapered in accordance with known practice ifdesired. A pair of dielectric strips can obviously be used on eitherside of a center conductor to feed a pair of conductively boundedchannels.

In FIG. 4 however a further modification of the invention is shown inwhich a single ferrite channel 16 is employed between ground plane 11and center divider 15. The space between divider 15 and the other groundplane is preferably filled with a block of conductive material 42,appropriately set back from the end of channel 16 to allow the wavefields to reform.

The principles of the invention are by no means limited to symmetricalstrip lines. As illustrated in FIG. 5 and the transverse cross sectionthereof in FIG. 5A these principles are applicable to the unsymmetricalform, sometimes referred to as Microstrip, in which a single groundplane 51 is related to a spaced conductive strip 52. As in FIG. 4,dielectric transition members 53 and 54 are employed to couple the stripto and from the conductively bounded channel 55. It should be apparentthat the performance of ferrite member 56 is essentially the same aseither one of the elements 16 or 17 of FIG. 1.

In all cases it is to be understood that the above describedarrangements are merely illustrative of a small number of the manypossible applications of the principles of the invention. Numerous andvaried other arrangements in accordance with these principles mayreadily be devised by those skilled in the art without departing fromthe spirit and scope of the invention.

What is claimed is:

1. A nonreciprocal device for electromagnetic wave energy of givenfrequency comprising a channel of rectangular transverse cross sectionhaving a conductive boundary defining pairs of wide and narrow walls,said channel being substantially completely filled with gyromagneticmaterial, means for applying a steady magnetic field to said materialthat increases transversely across the extent of said wide walls of saidchannel from a first strength substantially below to one substantiallyabove that producing resonance in said material at said frequency, andmeans for applying said wave energy to said channel with the electricfield thereof polarized normal to said wide walls to propagate in adirection normal to said transverse cross section.

2. The device according to claim 1 wherein the point at which saidbiasing field has the value producing gyromagnetic resonance at saidfrequency falls on said wide walls between the center line thereof andone of the narrow walls thereof.

3. The device according to claim 2 wherein said means for applying saidwave energy is located on the same side of said center line thereof assaid first field strength.

4. The device according to claim 2 wherein said channel has crosssectional dimensions which are very small fractions of the free spacewavelength of said energy.

5. The device according to claim 2 wherein said means for applying saidwave energy comprises a thin strip of conductive material and a spacedconductive ground plane each substantially lying respectively in theplane of the wide walls of said channel.

6. The device according to claim 5 including a transition member of highdielectric constant material interposed between said strip and saidchannel.

References Cited UNITED STATES PATENTS 3,095,546 6/1963 Ayres et al33324.2 3,426,299 2/1969 Dixon 333-242 X PAUL L. GENSLER, PrimaryExaminer U.S. Cl. X.R. 33384

